Electricity - Structure of the Atom | PPT | web4study

Electricity – Structure of the Atom | PPT



Electricity - Structure of the Atom

 

Electricity

Topics Covered in this ppt

Negative and Positive Polarities

Electrons and Protons in the Atom

Structure of the Atom

The Coulomb Unit of Electric Charge

The Volt Unit of Potential Difference

Charge in Motion Is Current

Resistance Is Opposition to Current

The Closed Circuit

The Direction of Current

Direct Current (DC) and Alternating Current (AC)

Sources of Electricity

The Digital Multimeter

Electrons and Protons:

  • All the materials we know, including solids, liquids, and gases, contain two basic particles of electric charge: the electron and the proton.
  • The electron is the smallest particle of electric charge having the characteristic called negative polarity.
  • The proton is the smallest particle of electric charge having the characteristic called positive polarity.
  • The arrangement of electrons and protons in a substance determines its electrical characteristics.
  • When the number of protons and electrons in a substance are equal, they cancel each other out, making the substance electrically neutral.
  • To use the electrical forces associated with the negative and positive charges in the matter, the electrons and protons must be separated.
  • Changing the balance of forces produces evidence of electricity.

Electrons and Protons in the Atom

  • Electrons and protons in an atom assemble in specific combinations for a stable arrangement.
  • Each stable combination makes one particular kind of atom.
  • A hydrogen atom contains one proton in its nucleus. This is balanced by one orbiting electron. A hydrogen atom contains no neutrons in its nucleus.
  • Electrons are distributed in orbital rings around the nucleus.
  • The distribution of electrons determines the atom’s electrical stability.
  • The electrons in the orbital ring farthest from the nucleus are especially important.
  • If electrons in the outermost ring escape from the atom they become free electrons.
  • Free electrons can move from one atom to the next and are the basis of electric current.

Electrons and Protons in the AtomElectrons and Protons in the Atom 

Fig. 1-3: Atomic structure showing the nucleus and its orbital rings of electrons. (a) Carbon (C)  atom has 6 orbital electrons to balance 6 protons in the nucleus. (b) Copper (Cu) atom has 29 protons in the nucleus and 29 orbital electrons.

  • When electrons in the outermost ring of an atom can move easily from one atom to the next in a material, the material is called a conductor.
  • Examples of conductors include:
  • silver
  • copper
  • aluminum.
  • When electrons in the outermost ring of an atom do not move about easily but instead stay in their orbits, the material is called an insulator.
  • Examples of insulators include:
  • glass, plastic, rubber.

Semiconductors are materials that are neither good conductors nor good insulators.

Examples of semiconductors include:

  • carbon
  • silicon.
  • germanium

Atomic Number

  • The atomic number of an element is the number of protons in the nucleus of the atom balanced by an equal number of orbiting electrons.
  • The number of electrons in orbit around the nucleus of a neutral atom is equal to the number of protons in the nucleus.

Orbital Rings

  • Electrons are contained in successive rings beyond the nucleus. The rings are called K, L, M, N, O, P, and Q, respectively.
  • Each ring has a maximum number of electrons for stability. They are:
  • K ring = 2 electrons.
  • L ring = 8 electrons.
  • M ring = 8 or 18 electrons.
  • N ring = 8,18, or 32 electrons.
  • O ring = 8 or 18 electrons
  • P ring = 8 or 18 electrons
  • Q ring = 8 electrons
  • The maximum number of electrons in the outermost ring is always 8.
  • The electron valence of an atom is the number of electrons in an incomplete outermost shell. The valence indicates how easily the atom can gain or lose electrons.
  • An atom’s nucleus contains neutrons as well as protons.
  • Neutrons have no net electric charge.

The Coulomb Unit of Electric Charge

  • Most common applications of electricity require the charge of billions of electrons or protons.
  • 1 coulomb (C) is equal to the quantity (Q) of 6.25 × 1018 electrons or protons.
  • The symbol for electric charge is Q or q, for quantity.

Negative and Positive Polarities

  • Charges of the same polarity tend to repel each other.
  • Charges of opposite polarity tend to attract each other.
  • Electrons tend to move toward protons because electrons have a much smaller mass than protons.
  • An electric charge can have either negative or positive polarity. An object with more electrons than protons has a net negative charge (-Q) whereas an object with more protons than electrons has a net positive charge (+Q).
  • An object with an equal number of electrons and protons is considered electrically neutral (Q = 0C)

Physical force between electric charges Physical force between electric charges Physical force between electric charges

Fig. 1-5: Physical force between electric charges. (a) Opposite charges attract. (b) Two negative charges repel each other. (c) Two positive charges repel.

Charge of an Electron

  • The charge of a single electron, or Qe, is 0.16 × 10−18 C.
  • It is expressed
  • Qe = 0.16 × 10−18 C
  • (Qe indicates the charge is negative.)
  • The charge of a single proton, QP, is also equal to 0.16 × 10−18 C .
  • However, its polarity is positive instead of negative.

The Volt Unit of Potential Difference

  • Potential refers to the possibility of doing work.
  • Any charge has the potential to do the work of moving another charge, either by attraction or repulsion.
  • Two, unlike charges, have a difference of potential.
  • The potential difference is often abbreviated PD.
  • The volt is the unit of potential difference.
  • Potential difference is also called voltage.
  • The volt is a measure of the amount of work or energy needed to move an electric charge.
  • The metric unit of work or energy is the joule (J). One joule = 0.7376 ft·lbs.
  • The potential difference (or voltage) between two points equals 1 volt when 1 J of energy is expended in moving 1 C of charge between those two points.
  • 1 V = 1 J / 1 C

Charge in Motion Is Current

  • When the potential difference between two charges causes a third charge to move, the charge in motion is an electric current.
  • Current is a continuous flow of electric charges such as electrons.

Potential difference

Fig. 1-9: Potential difference across two ends of wire conductor causes drift of free electrons throughout the wire to produce electric current.

  • The amount of current is dependent on the amount of voltage applied.
  • The greater the amount of applied voltage, the greater the number of free electrons that can be made to move, producing more charge in motion, and therefore a larger value of current.
  • Current can be defined as the rate of flow of electric charge. The unit of measure for electric current is the ampere (A).
  • 1 A = 6.25 × 1018 electrons (1C) flowing past a given point each second, or 1A= 1C/s.
  • The letter symbol for current is I or i, for intensity.

Resistance Is Opposition to Current

  • Resistance is the opposition to the flow of current.
  • A component manufactured to have a specific value of resistance is called a resistor.
  • Conductors, like copper or silver, have very low resistance.
  • Insulators, like glass and rubber, have very high resistance.
  • The unit of resistance is the ohm (Ω).
  • The symbol of resistance is R.

The Closed Circuit

  • A circuit can be defined as a path for current flow. Any circuit has three key characteristics:
  1. There must be a source of potential difference (voltage). Without voltage, current cannot flow.
  2. There must be a complete path for current flow.
  3. The current path normally has resistance, either to generate heat or limit the amount of current.

Open and Short Circuits

  • When a current path is broken (incomplete) the circuit is said to be open. The resistance of an open circuit is infinitely high. There is no current in an open circuit.
  • When the current path is closed but has little or no resistance, the result is a short circuit. Short circuits can result in too much current.

Direction of the Current

  • With respect to the positive and negative terminals of the voltage source, the current has direction.
  • When free electrons are considered as the moving charges we call the direction of current electron flow. Electron flow is from the negative terminal of the voltage source through the external circuit back to the positive terminal.
  • Conventional current is considered as the motion of positive charges. Conventional current flows in the opposite direction from electron flow (positive to negative).

Direction of the Current

Fig. 1-13: Direction of I in a closed circuit, shown for electron flow and conventional current. The circuit works the same way no matter which direction you consider. (a) Electron flow indicated with a dashed arrow in the diagram. (b) The conventional current indicated with the solid arrow. (c)  Electron flow as in (a) but with reversed polarity of a voltage source. (d) Conventional I as in (b) but reversed polarity for V.

 

Direct Current and Alternating Current

  • Direct current (dc) flows in only one direction.
  • Alternating current (ac) periodically reverses direction.
  • The unit for 1 cycle per second is the hertz (Hz). This unit describes the frequency of reversal of voltage polarity and current direction.

Direct Current and Alternating Current

 

Fig. 1-14: Steady dc voltage of fixed polarities, such as the output of a battery. Note the schematic symbol at left.

Fig. 1-15: Sine-wave ac voltage with alternating polarities, such as from an ac generator. Note the schematic symbol at left. The ac line voltage in your home has this waveform.

Sources of Electricity

  • All materials have electrons and protons.
  • To do work, the electric charges must be separated to produce a potential difference.
  • The potential difference is necessary to produce current flow.

Common sources of electricity include:

  • Static electricity by friction
  • Example: walking across a carpeted room
  • Conversion of chemical energy
  • wet or dry cells; batteries
  • Electromagnetism
  • motors, generators
  • Photoelectricity
  • materials that emit electrons when light strikes their surfaces; photoelectric cells; TV camera tubes

The Digital Multimeter

  • A digital multimeter (DMM) is a device used to measure the voltage, current, or resistance in a circuit.

The Digital Multimeter


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